High temperature solid electrolyte fuel cells convert chemical energy into direct current electrical energy, through an exothermic electrochemical reaction, typically at temperatures above 800.degree. C. The reaction takes place at the electrode-electrolyte interfaces where the electrolyte is sandwiched between an anode and a cathode. The reaction usually involves a relatively pure fuel, for example a mixture of hydrogen and carbon monoxide, and an oxidant such as oxygen or air. Where hydrogen and carbon monoxide fuel is utilized in a fuel cell system, it is provided from a reformer, upstream of a fuel cell stack. The external reformer reacts, for example, hydrocarbons, natural gas, or alcohols with steam in an endothermic process to produce a fuel suitable for the fuel cells, such as hydrogen and carbon monoxide mixtures. One generator system, utilizing such externally reformed fuels is taught by Isenberg, in U.S. Pat. No. 4,395,468.
The endothermic reaction at the external reformer requires substantial heat input. If excess heat from the fuel cell generator is used, in an external reformer, complicated refractory ducting systems would be required. Somers et al., in U.S. Pat. No. 4,374,184 sought to alleviate the inefficiencies of external reformers by providing in-situ reforming of fuel feed, such as methane mixed with steam, on an electrically inactive portion of a tubular fuel cell, as shown in FIG. 1 of the drawings.
In the prior art apparatus of FIG. 1, the fuel cell generator 10 includes a gas tight housing 12 with interior thermal insulation 22 surrounding an oxidant inlet chamber 18, a preheating chamber 16 with exhaust gas outlet 28, generating chamber 14 and fuel inlet and distribution chamber 19. Oxidant, such as air, enters inlet 26, flows through conduits 20 into the interior of fuel cells 40, reverses direction inside the fuel cell and exits into preheating chamber 16. Fuel feed, such as a CH.sub.4 +steam mixture, enters distribution chamber 19, flows through perforated wall 30 and is reformed at the electrically inactive portion 44 of the fuel cells 40. Thermal energy for this endothermic reaction is carried by the airstream. The reformed gas, now containing H.sub.2 +CO, then reacts at the active portions 46' of the fuel cells before passing, as depleted fuel, into the preheating chamber 16 to combust with the depleted oxidant, to heat incoming oxidant passing through conduits 20.
The diffusion of reformable fuel, such as methane, from the fuel feed, i.e., the transverse diffusion velocity, to the fuel cell wall is very high, so that the reformable fuel concentration in the fuel feed will decrease rapidly along the fuel cell length if catalysis at the fuel cell wall is efficient. Usually, all the reformable fuel needed for the full length of the fuel cell will be reformed in the first 1/3 of the each fuel cell length.
The exterior of the inactive fuel cell ends 44 contains catalyst, such as nickel, effective to reform the fuel gas at elevated temperatures. These "inactive" ends, i.e., inactive to generate power but capable of reforming gases, can be made without an underlying air electrode, or bare of electrical contacts to the air electrode. As can be seen, however, a large portion of active electrode length is lost, unless the entire generator is made longer, adding to material costs and space requirements. Also, the reforming reaction is endothermic, cooling the inactive end of the fuel cell from 100.degree. C. to 250.degree. C. below the active end; which could cause thermal stresses on the fuel cell tubes, which are generally of a layered ceramic design, with possible resultant cracking, unless temperature controls are carefully monitored. What is needed then is a more effective means to in-situ reform fuel feed in a fuel cell generator.